This invention relates to a method for preparing polyarylene sulfides. More particularly, this invention concerns itself with a method for synthesizing polyphenylene sulfides and related polymers for use as thermally stable laminatng resins.
Although many thermally stable polymers can be fabricated into fibers and films, the use of these materials as resins in laminating, molding or coating applications, often poses considerable processing and handling problems. Besides the required stability towards oxidative and hydrolytic degradation, polymers which are to be used as laminating resins must be low melting or soluble in inert solvents in order to wet the surface of the coated or reinforced material and should show good adhesive properties. If the final polymeric resin is a crosslinked material, the crosslinking process should not be accompanied by the evolution of any gaseous or other volatile products as these would cause voids that result in weak points in the final laminate or coating.
In an attempt to overcome the problems associated with the use of film forming resins, it was suggested that polyarylene sulfides be utilized for laminating applications. Polyphenylene sulfides and their related polymers are well known and have been previously reported in the literature. These polymers, however, did not possess either the solubility or the low melting ranges required for a laminating resin and thus the need for a polyphenylene sulfide with the proper properties became obvious. After considerable effort, a method has been evolved that provides the means for synthesizing polyarylene sulfides that possess the characteristics needed to make them useful as molding, coating and laminating resins. These resins are obtained in the precuring stage as polyarylene sulfides with and without substituted cyano groups along the polymer chains, and having softening ranges around 100.degree. C. The resins are cured by heating at temperatures between 350.degree. and 400.degree. C or by heating up to 290.degree. C under nitrogen in the presence of zinc chloride. It is theorized that the crosslinking might proceed by the trimerization of the nitrile groups without the formation of any volatile side-products. The products were determined to be crosslinked on the basis of their insolubility in hexamethylphosphoric triamide in which the non-crosslinked polymers were quite soluble and from infrared spectra that were similar to those obtained for the non-cured polymers without the nitrile absorption.